1. Field of the Invention
The present invention relates to a direction indicating device for displaying direction of a vehicle.
2. Description of the Related Art
An elevated bridge, an iron plate covering over a drain trench which is buried in a road, and the like, are made of magnetic materials such as iron frames and iron bars. Thus, when a vehicle runs on the iron plate over the drain trench and the elevated bridge, there is a case where a direction indicating device that detects geomagnetism to determine a direction of vehicle takes in the disturbance of geomagnetism caused by the above mentioned magnetic materials (hereinafter referred to as external disturbance) to cause an error in the determined direction of vehicle.
Then, a conventional direction indicating device finds an amount of external disturbance from the detected value by a geomagnetic sensor, and when the external disturbance is large, it assigns a large weighting to the detected values of the geomagnetic sensor up to the previous times and when the external disturbance is small, it assigns a large weighting to the current detected value of the geomagnetic sensor thereby to find a mean value of the detected values of the geomagnetic sensor (see, for example, patent document 1).
[Patent document 1] Japanese Unexamined Patent Publication No. 5-280988, Paragraph number [0012] to [0052] and FIG. 1.
Since the conventional direction indicating device is structured in the above mentioned manner and it finds external disturbance from the detected values of the geomagnetic sensor and assigns the weighting to the detected values according to the amount of external disturbance, the conventional direction indicating device has a problem that if the conventional direction indicating device takes in large external disturbance when the vehicle is turning, because it assigns a large weighting to the detected values of the geomagnetic sensor up to the previous times, it degrades trackability in a display direction when the vehicle is turning.
In addition, when vehicles are different in type from each other, the geomagnetic sensors are mounted in different positions according to their types (for example, a distance from the ground to the geomagnetic sensor is different between a vehicle of the sedan type and a vehicle of the recreational vehicle type because a height of the vehicle is different between them) and when the geomagnetic sensors are mounted in the different positions, the detected values of the geomagnetic sensors become different from each other. Thus, the conventional direction indicating device has also a problem that it is required to assign suitable weighting to the detected values of geomagnetic sensor according to the position where the geomagnetic sensor is mounted.